Thermal management system for an electrical machine

ABSTRACT

A thermal management system for an electrical machine. The thermal management system has a housing circumferentially enclosing a bushing that has a tubular section. At least one fluid channel for a thermal medium is formed by channel walls within the housing and the bushing. A gap is provided or located between the tubular section of the bushing and the channel walls of the housing. The tubular section closes the at least one fluid channel off against a stator of the electrical machine, enabling a heat transfer between the stator and the thermal medium in the at least one channel across the tubular section of the bushing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This specification is based upon and claims the benefit of priority fromUnited Kingdom patent application number GB 2110346.0 filed on Jul. 19,2021, the entire contents of which is incorporated herein by reference.

BACKGROUND Technical Field

The disclosure relates to a thermal management system for an electricalmachine.

Description of the Related Art

Electric machines are increasingly used e.g. in automotive or inaeronautical applications. In particular, in cases of high voltage andhigh power applications, the electric machines generate considerableamounts of heat which has to be removed efficiently from the machines.

A liquid cooling system for this purpose is e.g. known from U.S. Pat.No. 7,948,126 B2, which describes a cooling jacket round the electricmachine.

As electric machines are stored and/or operated under a wide range ofthermal conditions, thermal management systems reducing thermal stresseswithin the electric machine are required.

There is a need for a thermal management system that addresses at leastsome of these issues or at least provides a useful alternative.

SUMMARY

In a first aspect there is provided a thermal management system for anelectrical machine. The thermal management system has a housingcircumferentially enclosing a bushing that has a tubular section. Atleast one fluid channel for a thermal medium is formed by channel wallswithin the housing and the bushing. A gap is provided or located betweenthe tubular section of the bushing and the channel walls of the housing.The tubular section closes the at least one fluid channel off against astator of the electrical machine, enabling a heat transfer between thestator and the thermal medium in the at least one channel across thetubular section of the bushing.

The gap prevents the accumulation of mechanical stress, by providingsome expansion space under thermal loads.

The channel wall can be some sort of protrusions extending radially fromthe housing and/or the bushing. The channel walls can be in one piecewith the housing and/or the bushing or they can be connected (e.g.welded) to the housing and/or bushing.

Embodiments of the thermal management system can be used in particularfor electrical machines with a high power-to-weight ratio, which aree.g. used in aircrafts. Generally those embodiments can be used forelectrical machines with a radial heat flux.

In some embodiments, the stator of the electrical machine comprises awinding which radially encloses a lamination stack. As the thermalbehavior of the lamination stack is critical, the arrangement togetherwith the enclosing tubular section of the bushing is beneficialregarding the thermal stress. In particular, for an embodiment in whichthe bushing, in particular the tubular section of the bushing is lessstiff than the lamination stack. Stiff in this context means that themaximal or mean radial deformation of the bushing, in particular theenclosing tubular part of the bushing is less than the maximal or meanradial deformation of the lamination stack under the same operatingcondition or load. The lamination stack can comprise Co—Fe or Si—Fe asmaterials.

In some embodiments, the tubular section of the bushing has a smoothsurface, which allows for a relatively easy deformation (i.e. lessstiffness).

One way of reducing the stiffness is a thin walled bushing, e.g. with aradial wall thickness of the bushing in the range of 0.8 to 0.3 timesthe thickness of the wall of the housing, in particular in the range of2 to 5 mm, in particular 3 mm.

The tubular section of the bushing closes the at least one fluid channeloff against a stator (located radially inwards from the bushing) of theelectrical machine, enabling a heat transfer between the stator and thethermal medium in the at least one channel across the tubular section ofthe bushing.

For improved stress properties, the linear thermal expansioncoefficients of the materials of the housing and the bushing, inparticular the tubular part of the bushing differ only by 10% or less.This means that both materials will deform under thermal load more orless the same. If the housing and/the bushing comprises or are made fromthe same material, in particular aluminum, both parts will deform in thesame way under the thermal load.

In some embodiments, the connection between the housing and the bushingcomprises at least one seal, in particular an O-ring seal, and/or atleast one welding connection. In particular, the at least one seal andthe at least one the welding connection are of identical design anddimension. Again, this allows for better thermal stress distributionwithin the electrical machine. A further improvement in that directioncan be made, if the at least one seal and/or the at least one weldingconnection are axially located towards the axial ends of the housing, inparticular axially outside the terminal channels (i.e. axially outermostchannels). This implies that the heat transfer is not impeded by theseals and/or the welding connections because they lie axillary outsidethe main heat transfer area of the bushing.

In some embodiments, a torque bearing connection between the laminationstack and the bushing is made through an interference fit.

In some embodiments, the yoke of the lamination stack has a radialthickness in the range of 1 to 5 times the thickness of the tubularsection of the bushing. This means that the thickness of the yoke isrelatively thin resulting in an improved heat transfer.

In some embodiments, the housing and the bushing, in particular thetubular section of the bushing, are in one part. In particular thehousing and the bushing, in particular the tubular section of thebushing, are in one part, but comprising different materials. This canbe manufactured through an additive manufacturing process.

In some embodiments, the thermal medium is selected from water, oil andair.

Thermal loads during operation are typically dynamic. This might resultin changing size of the gap.

In some embodiments, a control system is used for adjusting the gapbetween the housing and the bushing by controlling a flow property ofthe thermal medium, in particular the flow rate and/or temperature. Ife.g. the gap is reduced, the cooling load could be increased tocounteract the gap closing. The control system may include a sensor thatconfigured to detect the width of the gap, and when the gap is below athreshold value, the cooling load is automatically changed by thecontrol system to bring back the gap to a desired size.

Embodiments of the thermal management system can be applied, forexample, to an electrical motor or an electrical generator.

In a second aspect there is provided an electrical machine that has athermal management system of the first aspect.

In a third aspect there is provided an aircraft that has at least oneelectrical machine of the second aspect.

The skilled person will appreciate that except where mutually exclusive,a feature or parameter described in relation to any one of the aboveaspects may be applied to any other aspect. Furthermore, except wheremutually exclusive, any feature or parameter described herein may beapplied to any aspect and/or combined with any other feature orparameter described herein.

Throughout this specification and in the claims that follow, unless thecontext requires otherwise, the word “comprise” or variations such as“comprises” and “comprising”, will be understood to imply the inclusionof a stated integer or group of integers but not the exclusion of anyother stated integer or group of integers.

DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only, with referenceto the Figures, in which:

FIG. 1 shows a schematic half-section of an electrical machine with anembodiment of a thermal management system of the present disclosure;

FIG. 2 shows a sectional view of an embodiment of the thermal managementsystem of the present disclosure;

FIG. 3 shows a detail view of a lamination stack in an embodiment of thethermal management system of the present disclosure;

FIG. 4 shows a second embodiment of the thermal management system of thepresent disclosure that differs from the embodiment shown in FIG. 2 byhaving a bushing that is stiffer than the bushing in the embodimentshown in FIG. 2 .

FIG. 5 shows a third embodiment of the thermal management system of thepresent disclosure that includes a sensor.

The following table lists the reference numerals used in the drawingswith the features to which they refer:

Ref no. Feature FIG.  1 housing of electrical machine 1 2 4 5  2 bushing1 2 4 5  2′ tubular section of bushing 2 3  2″ radial section of bushing2  3′ first seal 2  3″ second seal 2  4 winding 2  5 lamination stack 23  6 connection between bushing and housing 2  7 interference fit 2 3  8gap between housing and bushing 1 2 5  9 channel for thermal managementmedium 1 2 4 5 9′, 9″ terminal channels (in axial direction) 2 10 yoke 311 channel walls 1 2 12 protrusions 4 15 sensor 5 20 electrical machine1 4 5 21 stator 1 4 5 22 rotor 1 4 5 23 shaft 1 4 5 30 indirect thermalmanagement system 1 2 4 5 40 control system 5 D1 thickness (in radialdirection) of the yoke of the 3 lamination stack D2 thickness (in radialdirection) of the tubular part 3 of the bushing Q heat flux 2

DETAILED DESCRIPTION

Aspects and embodiments of the present disclosure will now be discussedwith reference to the accompanying figures. Further aspects andembodiments will be apparent to those skilled in the art.

In FIG. 1 an electrical motor 20 is shown in a half-sectional view.Inside a housing 1 of the electrical motor 20, a rotor 22 and a stator21 are operating in a generally known way. During operation a shaft 23is rotated, generating shaft work from electrical power.

During operation, the electrical motor 20 also generates heat Q whichgenerally flows radially outward in the electrical machine 20.Therefore, an indirect thermal management system 30 is located on theradial outside of the electric motor 20. Indirect thermal managementmeans that a thermal fluid (e.g. air, cooling water) is not in contactwith the internals of the electrical motor 20. In principle (ifoperation conditions require this), it is also possible to use thethermal management system 30 for heating, i.e. the heat flow Q would bereversed.

In the following, aspects of a cooling effected by the thermalmanagement system 30 are described.

For this purpose, the thermal management system 30 comprises a number ofchannels 9 in the housing 1 for circulating a thermal medium, here afluid cooling medium such as e.g., water, oil or air. The inlets andoutlets for the cooling medium are not shown here for the sake ofsimplicity.

In the embodiment shown, the channels 9 are parallel to each otherextending circumferentially around the tubular section 2′ of the bushing2. The channels are annular and thus tight. In other embodiments, thechannels 9 might a different form and/or orientation (e.g., they areinclined against the rotational axis). The channels 9 are here formed ormachined into the material of the housing 1. In a further embodiment(see FIG. 4 ), the channels 9 are formed by protrusions from the bushing2 and the housing 1.

In the embodiments shown, the channels 9 are walled off by walls 11 ofthe housing, the walls 11 protruding radially inwards and by a bushing 2which generally comprises a thin-walled tubular section 2′. The tubularsection 2′ of the bushing 2 closes the channels 9 off from the stator21.

A gap 8 between the ends of the walls 11 and the bushing 2 allows forsome expansion space under thermal loads.

It should be noted that the electrical motor 20 is only an example foran electrical machine. Other electrical machines 20 which can be used inconnection with the embodiment of a thermal management system 30 cane.g. be generators.

In FIG. 2 details of an embodiment of a thermal management system 30 areshown, comprising a cooling jacket for indirectly cooling the electricmachine 20 with a cooling medium.

The thermal management system 30 comprises the housing 1 of theelectrical machine 20 and the bushing 2. The bushing 2 comprises thealready mentioned tubular section 2′ which is concentrically locatedwithin the housing 1 and a radial section 2″ which extends radiallyoutwards.

Between the housing 1 and the bushing 2 two seals 3′, 3″ prevent theleaking of the fluid cooling medium.

A first seal 3′ comprises a radially sealing O-ring which is positionedin a circumferential groove on the inside of the housing 1. The sealingis against the tubular section 2′ of the bushing 2.

A second seal 3″ is also an O-ring, but positioned in axial groove inthe housing 1, providing an axial seal against the radial section 2″ ofthe bushing.

In other embodiments different types of seals can be used. The housing 1and the bushing 2 are connected through a bolt connection 6 which isshown here only in exemplary manner.

In other embodiments the connection between the housing 1 and thebushing 2 can be made by welding connections.

The stator 21 comprises a winding 4 and radially outside from thewinding 4, a lamination stack 5. The lamination stack 5 transfers torqueto the bushing 2—located radially outwards from the lamination stack5—typically through an interference fit 7.

In an application with a high power-to-weight-ratio, the housing 1 canbe made from aluminum as this lightweight. It also has a much higherthermal expansion coefficient than the lamination stack 5. This meansthat under operation the thermal load on the housing 1 will radiallyexpand outwards faster than the lamination stack 5.

In case of an interference fit, a pre-stress will be introduced betweenthe lamination stack 5 and the bushing 2 during assembly. The stress inthis connection will increase or decrease due to the rather widetemperature variations.

To ensure an efficient heat transfer from the internal parts of theelectrical machine 20 radially outwards, the lamination stack 5 and thebushing 2 should remain in physical contact through the interference fit7, i.e., no gap should occur.

For an efficient heat transfer, the bushing 2 is designed to be lessstiff than the housing 1. Here the heat transfer takes part across thetubular section 2′ of the bushing 2. The tubular section 2′ of thebushing comprises a thin wall which is smooth on both sides. Inparticular, no protrusions extend from the tubular section 2′ as theywould stiffen it, in particular relative to the lamination stack 5. Thereduced stiffness in the tubular section 2′ of the bushing 2 results ina reduced thermal stress in the lamination stack 5. This enables athermal expansion of the bushing 2 (i.e. the tubular section 2′) and thelamination stack 5 system which is governed by the thermal expansion ofthe lamination stack 5.

By designing the tubular section 2′ of the bushing 2 less stiff, thethermal stress can be reduced, so that materials more optimized fortheir electromagnetic properties can be used in the electric machine 20.

The channel walls 11 in this embodiment are solely present in thehousing 1, making the housing 1 relatively stiff.

In one embodiment, a gap 8 between the housing 1 and the tubular section2′ of the bushing 2 is provided to ensure that the stiffness of thebushing 2 will not change due to a contact with the housing 1, inparticular with the protruding channel walls 11. This means that thetubular section 2′ preferably should not be in contact with the housing1 under in all operation conditions and/or during storage.

In one embodiment, the materials of the housing 1 and the bushing 2, inparticular the material of the tubular section 2′, are the same (e.g.aluminum). Therefore, the linear thermal expansion coefficients of bothparts are the same, reducing the thermal stresses overall. In otherembodiments, the materials can be different, but the linear thermalexpansion coefficients should not differ by more than 10% (or less) toensure limited thermal stresses under operation.

Generally, it is possible that the bushing 2 can comprise or is made ofsteel, Inconel or titanium.

A further design feature to reduce additional thermal stresses at theconnection between the housing 1 and the bushing 2 (e.g. the boltconnection 6), the seals 3′, 3″ should be located reasonably far fromthe lamination stack 5. In the embodiment shown, the seals 3′, 3″ areaxially located outside the axial first terminal channel 9′ (axiallyleftmost) and outside the axially last terminal channel 9″ (axiallyrightmost), i.e., the seals are axially located outside the axial endchannels 9′, 9″.

Under operation, there will be thermal stress in the bushing 2, inparticular the tubular section 2′ of the bushing. But by keeping thetubular section 2′ smooth, the stress concentration is reduced.

In one embodiment of the thermal management system 30, a yoke 10 of thelamination stack 5 (see FIG. 3 ) is relatively thin. The radialthickness D1 of the yoke 10 should be in the range of 1 to 5 times theradial thickness D2 of the tubular section 2′ of the bushing 2.

The embodiments allow in high power-to-weight ratio electric machines 20a better choice of materials, in particular for the lamination stack 5,as the thermal stress issue is addressed by the design of the bushing 2.By choosing the materials not for their thermal properties but rathertheir mechanical or electrical properties, the overall weight of theelectrical machine and the efficiency can be improved.

In the embodiments shown in FIGS. 1 to 3 , the housing 1 and the bushing2 were two separate parts. In a different embodiment the housing and thebushing 2 can be in one piece, e.g. manufactured by a casting process oran additive manufacturing process. In latter, it would be even possibleto print the bushing 2 and the housing 1 with different materials.

These embodiments can be used in electrical machines under a widetemperature range for example between −65 to 120° C. This reducedthermal stress on the lamination stack allows the use of optimizedmaterial resulting an improved overall efficiency of the electricalmachine.

In FIG. 4 an embodiment is shown that uses a bushing 2 which is somewhatstiffer than the bushing 2 in the embodiment shown in FIG. 2 .Otherwise, the above description is applicable. Here, some protrusions12 extend radially outwards from the tubular section 2′ of the bushing.

In FIG. 5 a further variation of the embodiment shown in FIG. 1 isshown. As mentioned above, the gap 8 between the bushing 2 and thehousing 1 prevents the built-up of mechanical stress between the bushing2 and the housing 1. In the embodiment shown, a sensor 15 detects thewidth of the gap 8 e.g. by using a capacitive measurement, a resistancemeasurement or a change in mechanical vibrations (e.g. through apiezoresitive sensor). If the gap 8 is e.g. below a certain thresholdvalue, the cooling load is automatically changed by a control system 40to bring back the gap 8 to the desired size.

It will be understood that the disclosure is not limited to theembodiments above-described and various modifications and improvementscan be made without departing from the concepts described herein. Exceptwhere mutually exclusive, any of the features may be employed separatelyor in combination with any other features and the disclosure extends toand includes all combinations and sub-combinations of one or morefeatures described herein.

We claim:
 1. A thermal management system for an electrical machine, thethermal management system comprising a housing circumferentiallyenclosing a bushing that has a tubular section, wherein at least onefluid channel for a thermal medium is formed by channel walls within thehousing and the bushing, with a gap being provided or located betweenthe tubular section of the bushing and the channel walls of the housing,and the tubular section closes the at least one fluid channel offagainst a stator of the electrical machine, enabling a heat transferbetween the stator and the thermal medium in the at least one channelacross the tubular section of the bushing.
 2. The thermal managementsystem of claim 1, wherein the stator includes a winding which isradially enclosed by a lamination stack.
 3. The thermal managementsystem of claim 1, wherein the bushing is less stiff than the laminationstack.
 4. The thermal management system of claim 1, wherein a radialwall thickness of the bushing is in the range of 0.8 to 0.3 times thethickness of the wall of the housing.
 5. The thermal management systemof claim 1, wherein the maximal or mean radial deformation of thebushing is less than the maximal or mean radial deformation of thelamination stack under the same operating condition or load.
 6. Thethermal management system of claim 1, wherein the tubular section of thebushing has a smooth surface.
 7. The thermal management system of claim1, wherein the linear thermal expansion coefficients of the materials ofthe housing and the bushing differ by 10% or less.
 8. The thermalmanagement system of claim 1, wherein a connection between the housingand the bushing comprises at least one seal and/or at least one weldingconnection.
 9. The thermal management system of claim 8, wherein the atleast one seal and the at least one welding connection are of identicaldesign and dimension.
 10. The thermal management system of claim 8,wherein the at least one seal and the at least one welding connectionare axially located towards the axial ends of the housing.
 11. Thethermal management system of claim 1, wherein a torque bearingconnection between the lamination stack and the bushing made through aninterference fit.
 12. The thermal management system of claim 1, whereina yoke of the lamination stack has a radial thickness in the range of 1to 5 times the thickness of the tubular section of the bushing.
 13. Thethermal management system of claim 1, wherein the housing and thebushing are in one part.
 14. The thermal management system of claim 13,wherein the housing and the bushing are in one part but comprisedifferent materials as manufacturable through an additive manufacturingprocess.
 15. The thermal management system of claim 1, wherein thethermal medium is a cooling fluid selected from water, oil and air. 16.The thermal management system of claim 1, further including a controlsystem that adjusts the gap between the housing and the bushing bycontrolling a flow property of the thermal medium, the control systemoptionally including a sensor configured to detect the width of the gap,and when the gap is below a threshold value, the cooling load isautomatically changed by the control system to bring back the gap to adesired size.
 17. The thermal management system of claim 16, wherein theflow property of the thermal medium is flow rate or temperature.
 18. Thethermal management system of claim 1, wherein the electrical machine isan electrical motor or an electrical generator.
 19. An electricalmachine including a thermal management system of claim
 1. 20. Anaircraft including at least one electrical machine of claim 19.